Answer:
d. All of these statements are true.
Step-by-step explanation:
According to the "A Citizen's Guide to Radon" manual on the third page:
"Radon comes from the natural (radioactive) breakdown of uranium in soil, rock and water and gets into the air you breathe. Radon can be found all over the U.S. It can get into any type of building—homes, offices, and schools—and result in a high indoor radon level. But you and your family are most likely to get your greatest exposure at home, where you spend most of your time."
According to a study by Brazilian researchers, the greatest exposure to gas radon occurs indoors, such as tunnels, wineries, mines and homes. All of these environments are typical of developed countries that exploit environments to build cities and homes for the population.